A conventional hydraulic chain tension comprises a housing, a bottomed hollow cylindrical plunger slidable in a plunger-accommodating hole formed in the housing, a plunger biasing spring, which biases the plunger in a protruding direction, a high-pressure oil chamber formed by the plunger-accommodating hole and the plunger, and a check valve, incorporated into the housing, which allows oil to flow into the high-pressure oil chamber but blocks backflow of the oil.
In this type of the hydraulic chain tensioner, chain tension is held at a prescribed level by using a spring-biased plunger to exert a chain tensioning force, and by buffering the pressing force applied to the plunger from the chain by oil pressure in a high-pressure oil chamber formed by the plunger and the housing.
If the engine is started while the high-pressure oil chamber is insufficiently filled with oil (for example, if the engine is started after having been out of operation for a long time), it is difficult to maintain tension in the timing chain over the time interval during which oil, supplied from an oil pump, fills the high-pressure oil chamber of the tensioner. During this interval, if the chain exerts a significant pushing force on the plunger of the tensioner, backlash and fluttering of the chain can occur.
To address this problem, a chain tensioner, which can restrict retracting movement of the plunger, has been proposed in U.S. Pat. No. 6,935,978. The proposed tensioner comprises a bottomed cylindrical housing, a plunger slidable in the housing, a biasing spring, which exerts a force urging the plunger in the protruding direction, a plurality of engagement grooves formed on the outer circumference of the plunger, a first stopper formed on an inner circumference of the housing, and register rings, which are elastically expandable and retractable, and which engage the engagement grooves, to restrict backward movement of the plunger by causing an engagement groove to engage with the first stopper through register rings.
In my U.S. patent application Ser. No. 11/083,411, filed Mar. 18, 2005, I describe a hydraulic chain tensioner having a plunger, a housing, a biasing spring, and a check valve, in which backward movement of the plunger is restricted by an expansible C-shaped ring, disposed in an annular groove formed on the outer circumference of the plunger, with predetermined clearances in the groove width direction and in the radial direction between the ring and the groove bottom
The chain tensioner proposed in U.S. Pat. No. 6,935,978 includes a plurality of engagement grooved on the outer circumference of the plunger. Furthermore, a first stopper is formed on an inner circumference of the housing. To form the first stopper, an annular guide groove is formed on an inner circumferential surface of the housing. That is, the first stopper and the second stopper are respectively formed on both axial ends of the guide groove. Taking accuracy and other factors into consideration, significant manufacturing costs are incurred in forming the annular guide groove on the inner circumferential surface of the housing. Furthermore, since this chain tensioner restricts backward movement of the plunger by causing one of a plurality of engagement groove to engage the first stopper through a register ring, difficulties are encountered in designing conditions such as the shape and material of the register ring.
The ring type hydraulic tensioner disclosed in the specification of my application Ser. No. 11/083,411 prevents backlash and fluttering of a chain upon engine start-up when the hydraulic tensioner is not sufficiently filled with oil, by taking advantage of friction between and expansible C-shaped ring and the inner circumferential wall of a plunger-accommodating hole in a tensioner housing. However, because the C-shaped ring is made of steel, the inner circumferential wall of the plunger-accommodating hole can become worn by sliding friction. As wear of the inner circumferential wall of the plunger-accommodating hole proceeds, the clearance between the inner circumferential surface of the plunger-accommodating hole and the outer circumferential surface of the plunger is increased, and the increase of this clearance impairs the holding capability of the ring. As a result, backlash or fluttering of the chain, although prevented initially, can occur eventually. If an aluminum alloy is adopted as the housing material for the purpose of weight reduction, frictional wear of the inner circumferential surface of the plunger-accommodating hole proceeds rapidly. Furthermore, powder generated by wear of the inner circumferential surface of the plunger-accommodating hole accumulates in the vicinity of the oil supply passage or the check valve and affects oil inflow, which can also cause backlash and fluttering of the chain.
An object of the invention, therefore, is to provide a hydraulic chain tensioner, which can reduce friction on the inner circumferential surface of the plunger-accommodating hole of a hydraulic chain tensioner, and which can reduce manufacturing costs and simplify tensioner design.